Mr. J. Croll on the Excentricity of the Earth's Orbit. 1 19 



their distribution, that many advantages would flow from having 

 their wave-lengths the same, and consequently the positions of the 

 nodes identical, notwithstanding the very different velocities with 

 which wave-pulses are propagated along them. 

 I am, Gentlemen, 



Your obedient Servant, 



Samuel Haughton. 



Trinity College, Dublin, 

 January 18, 1867. 



XVII. On the Excentricity of the Earth's Orbit, and its Physi- 

 cal Relations to the Glacial Epoch. By James Croll*. 



THE Philosophical Magazine for January 1866 contains 

 a Table of the values of the excentricity of the earth's 

 orbit for one million of years before and after the epoch a.d. 

 1800. From that Table it is seen that between 1,000,000 and 

 700,000 years ago there were three periods when the excen- 

 tricity reached a very high value. It appears highly probable 

 that if the glacial epoch resulted from an extreme condition of 

 excentricity, these three periods might be those of the boulder- 

 clay, when the country was completely covered with ice, an 

 opinion in which I am happy to find Sir Charles Lyell concurs f. 

 As the Table referred to gives the values only at epochs 50,000 

 years apart, in order to arrive at a more accurate knowledge of 

 the condition of the earth's orbit during those periods, I have 

 calculated the excentricity at epochs 10,000 years apart from 

 1,000,000 to 700,000 years ago. Mr. Stone found that about 

 210,000 years ago there was a period when the excentricity 

 reached the value of 0*0575 %. In order to include this period, 

 I have also given the excentricities from the present time back 

 for 250,000 years at periods 10,000 years apart. 



Column IV. gives the number of days that the length of 

 the winter exceeds that of the summer when the winter occurs 

 in aphelion. At present our winter is about eight days shorter 

 than our summer, but 850,000 ) 7 ears ago, when it occurred in 

 aphelion, it would be actually about thirty-five days longer than 

 the summer. 



Column V. gives the number of degrees Fahrenheit that the 

 midwinter temperature of our island would be lower than at 

 present. This column has been calculated by a method de- 

 scribed in the ' Reader' for December 9th, 1865, and which 



* Communicated by the Author. 



t Principles of Geology, new edition, vol. i, p. 296. 



X Phil. Mag. for June 1865 (Supplement). 



